The invention relates to a method for determining the temperature of a strand of material such as, a glass fibre or a wire. For example, glass fibres are produced in drawing towers which can be considerably taller than 30 meters. In this case, a glass fibre is drawn from a preform about 100 to 250 mm in diameter that is heated to melting temperature at a speed of almost 3,000 m/min. If it is to be used for today's data transmission routes for example, the glass fibre itself typically has a diameter in the region of 125 μm, which must be manufactured with accuracies in the nanometre range.
In this case, the drawing temperature of the glass fibre in the hot region of the drawing tower is of key importance. If possible, the temperature of the preform should be regulated such that, given variable drawing speed, exact diameters of the glass fibre are already achieved in the start-up process, in order to minimise rejects. The glass fibre runs through a cooling stretch in the lower region of the drawing tower, in which it is cooled as far as possible to a constantly low temperature of 70° C. for example. Such cooling stretches are often operated using helium, which is extremely expensive. Accordingly, unnecessary cooling must be prevented so as not to unnecessarily increase the operating costs of the drawing tower plant. For this reason also, an accurate knowledge of the temperature of the glass fibre is important. In addition, at the end of the cooling stretch, glass fibres are usually provided with a coating. A prerequisite for this coating is that the glass fibre be at a constant temperature in order to minimize fluctuations in the thickness and concentricity of the close-tolerance coating. Knots and constrictions can also arise in drawing towers due to a dripping effect if the temperature of the glass fibre does not correspond to the optimum temperature characteristics of the coating.
Attempts were made to determine the temperature of such moving strands, such as glass fibres or wires, by means of a thermal imaging camera. In this case, the strand to be measured is imaged by means of a lens system focused on the thermal imaging camera. The temperature is then determined from the maximum value of the measuring values detected by the thermal imaging camera. However, this procedure has not proven reliable in practice. A key problem is a quick movement of the strands to be measured in relation to the time constant of the pixels of the thermal imaging camera. For example, high-frequency vibrations of the glass fibres conveyed along their longitudinal axis inevitably occur in drawing towers for producing glass fibres. For example, glass fibres about 125 μm in diameter can easily sway by 1 millimetre both transversely and longitudinally to the measuring plane in a drawing tower. Typical thermal time constants of standard thermal imaging sensors are in the range of 10 milliseconds. Other movements during production or a non-orthogonal orientation toward the imaging thermal imaging camera cause measuring errors. During manufacture the strand can also be subject to defocusing, for example due to a slow wandering of the fibre away from the centre due to a preform possibly not standing exactly vertically. Depending on the glass fibre manufacturer, this may possibly be identified, but only corrected from a certain extent onwards.
Particularly problematic are the explanatory difficulties with strands to be measured, of which the temperature signal only stands out a little from the temperature signal of a background. This is the case with glass fibres made out of quartz glass for example, which emit thermal radiation in an infrared wavelength-range from approximately 7 to 14 μm, which is regularly interesting for thermal imaging cameras, almost like a black body (emissivity e=0.95). A further problem lies in the fact that the strands to be measured, for example metal wires or glass fibres, are of smaller dimensions than the optical resolution capacity of the imaging system of thermal imaging camera and lens system.
All these problems mean that currently, a sufficiently accurate and reliable non-contact temperature measurement of strands of the type described using thermal imaging sensors is not possible.
Starting from the prior art explained, the invention is therefore based on the object to provide a method of the aforementioned type, using which the temperature of strands, such as wires and glass fibres, can be measured accurately and reliably.